Quick answer
Auxiliary heat that is always on means either the thermostat is calling for backup too easily or the heat pump has stopped producing heat. Setbacks of more than 1–2 °C, an aggressive aux threshold or a missing lockout temperature cause the first. An iced coil, low refrigerant or a dead outdoor unit cause the second. Electric strips use two to three times the electricity for the same heat.
Auxiliary heat that is always on is a billing problem before it is a comfort problem, because the house stays warm while the meter spins. AUX on the thermostat means the backup heat source is running alongside or instead of the heat pump: electric resistance strips in the air handler, or the gas furnace on a hybrid system. Some aux time is normal in deep cold and during defrost. Aux on a mild day, or for hours at a stretch, is a setting or a fault.
Thermostat settings that call for aux heat too often
The most common cause is the thermostat, not the equipment. A heat pump warms a house slowly, and most thermostats add backup heat when the room sits more than about 1–2 °C below the setpoint or recovery is taking too long. A 4 °C overnight setback therefore triggers the strips every morning. Hold one temperature, or keep setbacks to 1–2 °C, and that trigger disappears.
Two settings in the installer menu matter more than the schedule. The aux lockout (ecobee labels it aux heat max outdoor temperature) forbids backup heat above a chosen outdoor temperature. The compressor lockout does the opposite and shuts the heat pump off below a chosen temperature.
Both should be set from the balance point, the outdoor temperature where the heat pump's output just matches the house's heat loss, and both need a working outdoor sensor or weather feed. Left at factory defaults, they often hand the load to the strips far earlier than a cold-climate unit needs.
When the heat pump itself is not producing heat
If the outdoor unit is not doing its share, the thermostat falls behind and calls aux continuously. The house stays comfortable, so the failure goes unnoticed until the hydro bill arrives. Walk outside while AUX is showing: the outdoor fan should be turning and the unit should sound like it is working.
- Iced coil: a unit encased in ice cannot absorb heat from the air. See why a heat pump ices up.
- Low refrigerant charge: the unit runs constantly but delivers lukewarm air, and the strips fill the gap.
- Failed outdoor unit: a tripped breaker, a dead capacitor or contactor, or an inverter fault leaves the outdoor unit silent while the air handler carries on.
- Stuck heat relay or sequencer: the strips stay energized whenever the blower runs, whatever the thermostat asks for.
- Thermostat set up as a conventional furnace system, or wired so the strips come on with the first stage of heat.
What constant aux heat costs on Ontario time-of-use rates
Electric strips turn one kilowatt-hour of electricity into one kilowatt-hour of heat, a COP of 1. A heat pump moves heat instead of making it and delivers two to three times as much per kilowatt-hour through most of an Ontario winter, a COP of 2 to 3. Residential strip banks are typically 10 to 20 kW. A 15 kW bank uses 15 kWh for every hour it runs; a heat pump delivering the same heat at a COP of 2.5 uses 6 kWh.
Time-of-use pricing makes the timing matter as well. The Ontario Energy Board sets time-of-use electricity prices province-wide, so the pattern is the same whether your bill comes from Burlington Hydro, Oakville Hydro, Milton Hydro or Alectra, and winter on-peak hours fall on weekday mornings and early evenings. A morning recovery from a deep setback runs the strips in the most expensive hours of the day.
Hybrid systems: aux heat that is gas by design
On a hybrid system the backup is the gas furnace, and the thermostat may show AUX whenever the furnace is running. Below the switchover temperature the heat pump is locked out on purpose and the furnace heats the house alone. That is the design, not a fault, and it is billed at gas rates, not strip rates. It only needs attention if the furnace is running on mild days, which means the switchover temperature is set too high or the heat pump has a fault of its own.
Check this yourself first, and when to call now
- Note the outdoor temperature each time AUX appears. In the coldest weather some aux time is expected; above freezing it should be limited to defrost and brief top-ups.
- Stop large setbacks for a week and see whether aux run time falls.
- Confirm the thermostat is not in EM HEAT, which runs the backup alone.
- Check that the outdoor unit runs when the thermostat calls for heat, and that its breaker is on.
- Replace the air filter: low airflow makes the heat pump trip on high pressure and hand the load to the strips.
- Look for a run-time report in the thermostat's app; many smart thermostats log aux hours by day.
- Call now if AUX runs while the outdoor unit is silent or iced over.
- Call now if the strips stay on with the thermostat satisfied or set to OFF, or you smell hot dust or burning at the registers.
- Call now if the breaker for the air handler or the strip bank trips.
- Call now if your electricity use has jumped with no change in the weather.
Correcting a setting takes a diagnostic visit at $90–$150; a failed part on the outdoor unit usually lands in the $150–$800 repair range, priced in writing first. Setting the lockouts from the equipment's capacity data and your house, not from the default, is part of that visit. Book heat pump repair, compare when emergency heat is the right call, see every heat pump answer, or contact the shop.